Enable USE_ZMIN_PLUG on SCARA example (#9126)

Marlin/src/config/examples/SCARA/Configuration.h

 // extra connectors. Leave undefined any used for non-endstop and non-probe purposes.
 #define USE_XMIN_PLUG
 #define USE_YMIN_PLUG
-//#define USE_ZMIN_PLUG
+#define USE_ZMIN_PLUG
 //#define USE_XMAX_PLUG
 //#define USE_YMAX_PLUG
-#define USE_ZMAX_PLUG
+//#define USE_ZMAX_PLUG
 
 // coarse Endstop Settings
 //#define ENDSTOPPULLUPS // Comment this out (using // at the start of the line) to disable the endstop pullup resistors